Dye-sensitized solar cells are attracting attention as photoelectric conversion elements that are low in cost and enable a high conversion efficiency to be obtained (see, for example, Japanese Unexamined Patent Application, First Publication No. H01-220380; and Michael Graetzel, Nature, United Kingdom, 1991, vol. 737, p. 353). Generally, in this type of photoelectric conversion element, a semiconductor electrode is constructed by forming a porous film with oxide semiconductor nanoparticles of titanium dioxide or the like on a transparent conductive substrate, and then causing a sensitizing dye to be provided in this porous film. This semiconductor electrode is used with a counter electrode made of conductive glass that has been sputtered with platinum, and the space between the two electrodes is filled by a charge transfer layer in the form of an organic electrolyte solution that contains oxidizing species and reducing species such as iodine and iodide ions.
The photoabsorption coefficient is increased by providing this semiconductor electrode with a porous film structure having a large specific surface with a roughness factor of 1000 or more. A photoelectric conversion efficiency with a photoabsorption coefficient of 10% or more has also been reported. It has also been predicted that the cost of dye-sensitized solar cells will be reduced to about ½ to ⅙ of the cost of silicon based solar cells that are currently used. Because dye-sensitized solar cells do not necessarily require complex, large-scale manufacturing facility and neither do they contain harmful substances, they have a strong possibility of becoming inexpensive, mass-produced solar cells that are capable of being widely used.
The transparent conductive substrate is generally one that has been prepared in advance by covering a glass substrate surface with a transparent conductive film of tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO), or the like using a technique such as sputtering or CVD. However, the specific resistance of ITO or FTO is of the order of 10−4 to 10−3 Ω·cm, which is about 100 times greater than the specific resistance of metals such as silver or gold. Consequently, commercially available transparent conductive glass has a high resistance, and when it is used for solar cells, in particular, when it is used for large surface area cells, there is a marked deterioration in the photoelectric conversion efficiency.
One technique of lowering the resistance of transparent conductive glass that has been considered is to form the transparent conductive layer (such as the ITO or FTO) thicker. However, the photoabsorption by a transparent conductive layer increases if the film is formed having a thickness large enough to allow a satisfactory resistance to be obtained, and the ratio of transmitted light deteriorates markedly. By this, the photoelectric conversion efficiency of the solar cell also deteriorates.
As a solution to this problem, investigations are currently underway into lowering the resistance of a substrate that is provided with the transparent conductive layer that is used as a photoelectrode of a solar cell by providing a metal circuit (wiring) layer that does not markedly impair the opening area ratio on the surface of the substrate (see, for example, Japanese Patent Application No. 2001-400593). When a metal circuit layer is provided on the surface of the substrate in this manner, in order to prevent corrosion of the metal wiring by the electrolyte solution and to prevent reverse electron transfer from the metal circuit layer to the electrolyte solution, it is necessary for at least surface of the metal circuit layer to be protected by some type of shielding layer. This shielding layer must cover the surface of the substrate completely.
FIGS. 26A and 26B show an example of a dye-sensitized solar cell. This dye-sensitized solar cell is provided with a working electrode 63 that is formed on top of an electrode substrate 61 by fine particles of an oxide semiconductor such as titanium oxide and that has an oxide semiconductor porous film 62 that is provided with a photo-sensitizing dye, and with a counter electrode 64 that is provided opposite this working electrode 63. An electrolyte layer 65 is formed between the working electrode 63 and the counter electrode 64 by filling this space with an electrolyte solution.
The electrode substrate 61 is constructed by forming a transparent conductive layer 611 that is made of tin-doped indium oxide (ITO), fluorine-doped tin oxide (FTO), or the like on a base material 610 that is a glass plate or the like. In order to improve the current collecting efficiency from the oxide semiconductor porous film 62, a lattice-shaped metal circuit layer 612 that is made of gold, platinum, silver, or the like is provided on the transparent conductive layer 611. Furthermore, in order to restrict problems such as output deteriorations caused by corrosion of the metal circuit layer 612 or by short-circuiting with the electrolyte layer 65 or by leak current (i.e., reverse electron transfer), and the like, the surfaces of the metal circuit layer 612 and the transparent conductive layer 611 are covered by a shielding layer 613 that is made of an oxide semiconductor such as ITO, FTO, titanium oxide, zinc oxide, or the like. Instead of the electrolyte layer 65, it is also possible to use a solid charge transfer layer 66 that is made of a p-type semiconductor or the like. When light such as sunlight enters from the base material 610 side, electromotive force is generated between the working electrode 63 and the counter electrode 64.
The formation of the shielding layer 613 is achieved by forming a film made up of an oxide semiconductor on the metal circuit layer 612 using a thin film forming method such as a sputtering method, a spray thermal decomposition method (SPD), or the like. However, because the surfaces of the transparent conductive layer 611 and the metal circuit layer 612 present profiles having minute bumps and irregularities such as voids, cracks, and particle boundaries, it is difficult for a dense shielding layer 613 to be formed uniformly, and there are cases in which uncovered portions in which the metal circuit layer 612 is exposed are generated by incomplete formation of the shielding layer 613. In this case, there is a reduction in the ability to suppress problems such as deteriorations in output that are caused by corrosion of the metal circuit layer 612, or that are caused by the generation of leak current due to reverse electron transfer from the metal wiring last 612 to the electrolyte layer 65, and the characteristics of the solar cell may be considerably impaired.
If the thickness of the coating of the shielding layer 613 is increased in order to suppress incomplete formation of the shielding layer 613, the transfer of photo electrons may be inhibited, and there is a reduction in the photo transmittance so that, contrary to expectations, there is a possibility that the photoelectric conversion efficiency will be reduced.
For example, when the metal circuit layer 612 is formed using a conductive paste that includes conductive particles such as fine metal particles and a bonding agent such as glass frit as the main components, from the viewpoint of the conductivity of the metal circuit layer 612, it is preferable that the compounding ratio of the bonding agent be low, however, minute and sharp bumps and irregularities and shadow portions such as voids and pinholes tend to be generated on the interior and surface of the metal circuit layer 612, so that forming a shading layer is difficult. If the compounding ratio of the bonding agent is increased, there may be a reduction in the conductivity of the metal circuit layer 612. Because of this, there may be a reduction in the current collecting efficiency, and the cell characteristics may be impaired markedly.
If the metal circuit layer 612 is not provided on the electrode substrate 61, and an attempt is made to collect current from the oxide semiconductor porous film 62 using only the transparent conductive layer 611, because the specific resistance of the FTO semiconductor or the like that forms the transparent conductive layer 611 is about 10−4 to 10−3 Ω·cm, which is about 100 times greater than the specific resistance of metals such as silver or gold, there is a significant reduction in the photoelectric conversion efficiency, particularly in the case of a cell having a large surface area. If the thickness of the transparent conductive layer 611 is increased in order to lower the resistance thereof, there is a marked deterioration in the light transmittance of the transparent conductive layer 611, and, once again, the photoelectric conversion efficiency is reduced.
As viewed in a direction along the side on which films are deposited, if there are portions that casts shadows on the metal surface of the substrate (for example, undercut of the circuit wall surface or the like), some portions may not be covered by the shading layer. Because these tend to cause corrosion of the circuit and reverse electron transfer to the electrolyte solution and the like, the cell characteristics may be impaired markedly. In particular, a sputtering method or a spray thermal decomposition (SPD) method is preferably used as the method for forming a commonly-used film such as FTO, ITO, TiO2 or the like as the shading layer, however, in these methods, it is extremely difficult to form a film uniformly on shadow portions. For example, if a circuit is formed using an additive plating method, in some cases the circuit wall surface is formed in a tapered shape due to the characteristics of the plating resist. If the bottom portion of the resist pattern remains like a trail, this portion becomes an undercut after the circuit has been formed. In this manner, it becomes difficult to form a thin film of a dense shielding layer on the surface of a metal circuit.
If an attempt is made to maintain an opening area ratio that does not considerably impair the light transmittance while providing a satisfactory conductivity, it is necessary for the metal circuit layer to have a certain height. Accordingly, when forming the metal circuit layer, the substrate surface has a large number of bumps and irregularities. Because of this, problems arise such as, for example, film thickness uniformity being reduced in the formation of a semiconductor porous film for a dye solar cell, and cracking or peeling of the film may arise in the portions in which the bumps and irregularities are located.
For example, in the case of a circuit formed by printing a paste whose main components are conductive particles and a glass frit binder, and then baking the circuit at about 500° C., because the compounding ratio of the glass frit is reduced in order that the fusion between conductive particles is not hindered and a high conductivity can be obtained, typically, abrupt bumps and irregularities or shadows such as voids and pinholes are generated on the surface or interior of the coated film, and it becomes extremely difficult to form a shading layer. Conversely, if the compounding ratio of the glass frit, which forms a binder, is increased in order to suppress these types of defects in the coated film surface, there is a marked reduction in the conductivity of the coated film, and there is a tendency for the circuit to not exhibit its normal functions.
As shown in FIG. 27, a transparent conductive film 72 having a thickness of about 1 μm is made of indium-doped tin oxide (ITO) or fluorine-doped tin oxide (FTO) or the like on the entire surface of a glass plate shown by reference numeral 71, so as to form conductive glass 73. An oxide semiconductor porous film 74, which is sensitized with a photo-sensitizing dye and includes minute particles of an oxide semiconductor such as titanium oxide, niobium oxide or the like, is formed on top of the transparent conductive film 72 of this conductive glass 73. Reference numeral 75 denotes a conductive glass that is to be a counter electrode. An electrolyte layer 76 is formed by filling the space between the counter electrode 75 and the oxide semiconductor porous film 74 with an electrolyte solution of a non-aqueous solution that contains a redox couple such as iodine and iodide ions. Instead of the electrolyte layer 76, it is also possible to provide a hole transporting layer made of a solid p-type semiconductor such as copper iodide, copper thiocyanate, or the like. In this dye-sensitized solar cell, when light such as sunlight enters from the conductive glass 73 side, electromotive force is generated between the transparent conductive film 72 and the counter electrode 75.
In an actual dye-sensitized solar cell, because a circuit electrode is formed on a transparent conductive film, and an oxide semiconductor porous film is provided on top of that, and the space between them is filled with an electrolyte solution that contains iodine or the like, the circuit electrode contacts the electrolyte solution via the oxide semiconductor porous film. As a result, there are cases in which leak current arises due to electrons flowing reversely from the circuit electrode to the electrolyte solution. This occurs because, when comparing the energy levels of the circuit electrode with that of the electrolyte solution, the energy level of the electrolyte solution is lower. Therefore, currently, leak current is obstructed by the formation of a barrier layer that is made up of a semiconductor material or an insulating material at an interface between the electrode circuit and the electrolyte solution. However, because the barrier layers are formed with a variety of film forming methods, the problem of pinhole arises. Therefore, methods for solving this pinhole problem are being investigated, however, in this case, it is extremely important for practical reasons that the manufacturing method is an inexpensive one in which the cost is increased greatly (see Published Japanese Translation No. H08-15097 of the PCT International Application).